With the times of highly computerized society in recent years, communication devices and equipment for satellite broadcasting and mobile radio communication tend to be digitized, and signals are tend to be processed at a higher speed. As a reinforcing material for a printed wiring board used therefor, conventionally, a glass fiber is used, and E glass is known as a commercially produced glass fiber of this kind.
Generally, when alternating current is applied to glass, the glass absorbs energy of the alternating current and absorbs it as heat. The dielectric loss energy absorbed is in proportion to a dielectric constant and a dielectric tangent determined by the component and structure of glass, and is expressed by the following equation, EQU W=kfv.sup.2 .times..epsilon. tan .delta.
wherein W is a dielectric loss energy, k is a constant, f is a frequency, v.sup.2 is a potential gradient, .epsilon. is a dielectric constant, and tan .delta. is a dielectric tangent.
The above equation shows that as the dielectric constant and the dielectric tangent increase and as the frequency increases, the dielectric loss increases.
As far as E glass is concerned, it has a dielectric constant of 6.7 and a dielectric tangent of 12.times.10.sup.-4 at a frequency of 1 MHz at room temperature, and a printed wiring board for which E glass is applied is not sufficient for satisfying the demands of an increase in density and signal processing at a higher speed. It is therefore desired to develop glass having a lower dielectric constant and a lower dielectric tangent than E glass, and one kind of glass called D glass has been developed. One example of D glass has a composition of SiO.sub.2 73%, Al.sub.2 O.sub.3 1.0%, B.sub.2 O.sub.3 22%, CaO 0.6%, MgO 0.5%, Li.sub.2 O 0.6%, Na.sub.2 O 1.2% and K.sub.2 O 1.1%, and for example, it has a dielectric constant of 4.2 and a dielectric tangent of 10.times.10.sup.-4 at a frequency of 1 MHz at room temperature.
However, D glass has poor meltability and is liable to cause striae and foams, and its glass fibers frequently undergo breaking at a spinning step. D glass therefore has a defect in poor productivity and poor workability. Further, D glass has another problem in that it is liable to undergo peeling from a resin of a printed wiring board due to its poor water resistance, so that no high reliability is attained when it is used in printed wiring boards.
JP-A-7-10598 discloses a glass having a composition of SiO.sub.2 50.0-60.0%, Al.sub.2 O.sub.3 10.0-18%, B.sub.2 O.sub.3 11.0-25.0%, CaO 0-10.0%, MgO 0-10.0%, MgO+CaO 1.0-15.0%, ZnO 0-10.0%, SrO 0-10.0% and BaO 1-10.0%. Since, however, the above glass contains, as an essential component, BaO which increases the dielectric constant, it is difficult to attain a sufficiently low dielectric constant. For attaining a low dielectric constant, it is inevitably required to decrease the content of BaO. The problem is that the glass has a high viscosity in this case so that it is poor in spinning workability. Further, BaO also has another problem that it decreases the life of a glass melting furnace since it is highly corrosive to a material of the glass melting furnace.